Electron discharge amplifier



April 3@, 1935. CARPENTER 11,999,318

ELECTRON DI SCHARGE AMPLIFIER Filed May 24, 1950 Patented Apr. 30, 1935 UNITED STATS ATNT OFFIQE' Application May 24, 193%, Serial No. 455,326 lin Great Britain June 5, 1929 6 Claims.

This invention relates to electron discharge tube systems, and especially to amplifiers for varying electric currents wherein use is made of electron discharge tubes.

In my British Patent No. 325,833, thermionic systems are set forth wherein a stage comprising two electron discharge tubes is arranged to take current in its anode circuits from a source of supply in phase opposition. The invention disclosed in said British patent consists broadly in obtaining the necessary phase opposition in electron discharge tube systems employing voltage swings in phase opposition by making use of the phase reversing properties of an electron dis-- charge tube provided with three or more electrodes instead of obtaining the phase opposition from centre-tapped coils or divided circuits usually emplcyed previously. Two similar discharge tubes having equal resistances in their anode cir cuits are disclosed in the said British patent and arranged to co-operate in the production of fluctuating voltages of opposite phase in such a manner that the tubes are operated with equal voltage amplitudes, the result being obtained by arranging that the input voltage is applied between the grid and filament of the first tube, and amplified fluctuating voltage, reversed in phase, is taken directly from the anode of the tube, while a fractional part of it, equal to the input voltage, is applied between the grid and filament of the co-operating tube. The -l ethod of effecting this disclosed in the said British patout is to take a connection from a tap in the anode resistance of the first tube to the grid of the can-operating tube and with a condenser inserted in that connection.

However, if such an arrangement has its anode circuits supplied from a fluctuating source of supply, as is the case'when the supply is taken from power mains, or if fluctuations arise in the current drawn by the power stage, it is clear that owing to the tapped arrangement and the connection therefrom, the low frequency pulsations are not balanced, and in order to eliminate noises due to this, it is necessary to provide comparatively thorough filtering in the main supply. However, in accordance with the present invention, instead of obtaining the necessary fraction or the potential in the anode circuit of the first tube for application to the grid of the second tube by the connection set forth in the said'British patent, this fraction may be applied by connecting a comparatively high non-inductive resistance I between the anodes of the two tubes and by, taking off from this resistance at a suitable tap a connection leading through a condenser to the grid of the second tube. A cross-connection for balancing purposes can be applied to this form of connection, for which purpose a second tap is taken in the said resistance between the two anodes from a point situated on the opposite side of the mid-point of the resistance to the first tap mentioned above, and the connection from the second tap is taken through a con denser to the grid of the first tube, which, of course, in all such cases has a grid leak resis ance just as the second tube has.

The invention is illustrated in the accompanying drawing, wherein:

Figure 1 is a circuit diagram of a complete amplifier having two similar electron discharge tubes connected to co-operate in producing fluctuating voltages in opposed phase as set forth in my said British Patent No. 325,833; and

Figure 2 is a circuit diagram of the corres sponding portion of a system to that shown in Figure 1, but illustrating a method of applying the desired fraction of the anode potential of the first tube to the grid of the second tube in accordance with the present invention.

Referring first'of all to Figure l, A and B are two thermionic tubes to the grids of which equal voltage swings but opposite in time phase are applied. The anode circuit of the tube A and the grid circuit of the tube B are coupled by means of a condenser t connected in the lead tothe grid of the tube B, while 3 is the resistance in the anode circuit of the tube A. The battery supplying the filament cathodes of both tubes is shown at 5; ii is the battery supplying the anode circuits of both tubes; and 'l is the grid bias battery for the tube A and also supplies the bias voltage to the grid of the tube B. 8 is a resistance connected to the grid of the tube B and actsas a grid leak resistance, and 5 is the resistance in the anode circuit of the tube 13 for coupling purposes.

The arrangement is such, therefora'that the voltage swing applied to the grid of the tube B is obtained from-the resistance 3 in the anode circuit of the tube A, and is therefore in phase opposition to the voltage swing applied to the grid of the tube A. If, however, the full voltage were taken from the anode of the tube A, it would be of greater amplitude than that applied to the grid of the tube A, assuming that the eiTective magnification of the latter is greater than unity., In order, therefore, that the voltage swing applied to the grid of the tube B should be equal to that applied to the tube A, the voltage applied 'to the grid of the tube B is shown as taken from an adjustable tapping point iii in the resistance 3. A tapping point 8 is also shown on the leak resistance 8. The whole of the voltage adjustment may be made at the point ill or at the point 8 or one may constitute a coarse adjustment and the other a fine or final adjustment. The balancing adjustment may be made as described in my said British patent.

The condenser M has nothing to do with the principle of the system, but is merely a safeguard to ensure that the mean potentials of the grids of the tubes A and B are identical.

The leads It and i2 constitute'the output from the paraphase tubes A, B, whereas the conductor l3 connecting the negative endsof the filaments of all the tubes, correspondsto the neutral point.

The conductor l l is taken through a condenser IE to the grid of the third tube C, and the conductor i2 similarly through a condenser H3 to a fourth tube D, the tubes C and D having grid leak resistances l7 and 88 respectively. These tubes are in their turn coupled by resistances i9, 2! and capacities 22 to two further tubes E and F. Therefore, the tubes C and D are in effect amplifying stages for the two potentials opposite in phase but equal in amplitude obtained in the unit constituted by the tubes A and B, and theypass on these equal and opposite potentials to the tubes E and F after equal degrees of amplification.

It will be noted that the grid-bias battery i is tapped, its full voltage being applied to the grids of the tubes E and F and a part of the voltage being applied to the grids of tubes A, B, C, D. The anode battery 6 common to all the tubes has its negative terminal connected to the cathode filaments of all the tubes, and the lead 23'fromits positive terminal branches, one branch going through a choke coil 23 to the anode of the tube D, and the other branchthrough a choke coil 25 to the anode of the tube F. As an example, a loud speaking telephone, shown diagrammatically as of the cone diaphragm type, is shown at 26. The tubes E and F are provided with grid leak resistances 2i and 28 respectively. It will be clear that the output of the opposed phase system is taken off from the choke coils 24,25, which in practice may be constructed in the form of a centre-tapped coil, but these coils might be replaced by coupling windings coupled to a circuit containing the'loud speaking tele phone 26 orto separate circuits each containing a separate loud speaking telephone. Yetagain, the choke coils 2 25 might each be replaced by the coils of separate loud speaking telephones having appropriate impedance. It willbe noted also that if no potential variation is being impressed upon the input terminals l, 2, a steady current will be taken out of the battery 6. When the amplifier is operating, however, fluctuations in the current from the battery 6 to the anode circuitsof the tubes E andF tend to cancel out. So far the system is exactly in accordance with my said'British patent. It is found, however, that if there'is impedance in the common anode source shown as the battery f6,'back couplingwi1l exist of either or both of the types described above.

In'Figure 2 an element of an amplifieris shown which incorporates, however, a different method of taking oif the necessary fraction of the potential in the anode circuit of the tube A to be applied to-the input circuit of the tubeB. Infact, a comparatively high non inductive resistance iil is connected directly between the anodes of the tubes A and B, and fro-m this a connection is taken oif at a tap 4i and is taken through a condenser 42 to the grid of the tube B, the latter being furnished with a grid leak resistance 8.

The tap 4B is to the left of the mid-point m of the resistance td, that is to say, on the side of the mid-point towards the anode of the tube A. If capacitative reactances and the shunting effect of the leak resistance 8 are neglected, the point ii should be such that the ratio of onehalf of the total resistance 38 to the portion of the resistance between the points 4| and m is equal to the effective magnification of the tube A. Thus the mid-point m of the impedance 40 remains at a substantially steady potential, and

therefore acts as an electrical fulcrum or pivot. If, therefore, the tubes A and B are of fairly high voltage amplifying power, as they would be in practice, the tap is close to the central point m. If then the tube B is replaced by a tube having similar but not identical characteristics and a somewhat smaller voltage magnification, the fulcrum 'm will move alongthe resistance 49 towards the tube B. Due to the closeness of the points 4! and m, however, a small percentage movement of the point m to the right will cause a very much greater percentage 'increase in the distance between 4| and m, with the result that the tap ll will have impressed upon it an increased percentage of the alternating voltage applied to the ends of the resistance This increase will compensate almost entirely for any decrease in the effective amplifying power of the tube B. In practice, it is found that considerable variation in the characteristics of the tube B must take place before there is any measurable difference in voltage generated at the anodes of the two tubes A and B. Clearly also if the tube B is replaced by a tube of greater amplifying power, automatic compensation will take place in a similar but opposite fashion. Furthermore, a change in the characteristics of the tube A affects both sides equally. This arrangement is particularly convenient when the impedances 3 and 9 consist of iron-cored choke coils instead of resistances.

In order that a symmetrical connection may be provided to avoid the need of any very thorough filtration of the mains a second tap, not shown in Figure 3, would be takenvin the resistance M! on the opposite or right-hand side of the central point in and displaced from that central point by the same distance as the point M is displaced from it. The connection from that point is taken through a condenser, which would be equal in capacity to the condenser 42, and then to the grid of the tube A. The grid leak resistance 38 for the grid of the tube Awill, of course, in this event bemade of the same value as the leak 8 of the tube B. Precautions should be taken that the system is not asymmetrical, due to the impedance in which the signal voltage is generated.

The invention is obviously not limited to the example illustrated in the drawing. For example, it may beapplied to amplifiers or repeaters for feeding the two wiresof a metallic circuit telephone line without the use of transformer couplings, whichhas notbeen practicable previously owing to the fact that the capacities to earth of the two wires would be rendered unequal due to thedirect connection ofbatteries and so forth to one.of them. Again, thecoupling resistances in theanodes may :be replaced -.by

other impedances, such as inductive resistances. When inductive resistances, which may have iron cores, are employed, the present invention provides an improved amplifier, owing to the tendency to balance out the more injurious effects due to the non-linear property of the iron.

I claim:

1. In an electron discharge system, the combination of an electron discharge tube having a control electrode and an anode, a source of voltage variations connected to said control electrode, a second electron discharge tube having a control electrode and an anode, a source of electrical supply connected in the output circuits of both of said tubes and a high impedance direct current path connected between the anodes of said tubes, the control electrode of said second mentioned tube being connected to derive its input potential from a tap in said path, said input potential being selected so as to be in opposed phase to said source of voltage variations and being dependent upon the varying potentials of the anodes of both of said tubes.

2. In an electron discharge system, the combination of an electron discharge tube having a control electrode and an anode, a source of voltage variations connected to said control electrode, a second electron discharge tube having a control electrode and an anode, a source of electrical sup-- ply connected in the output circuits of both of said tubes, 2. high impedance direct current path connected between the anodes of said tubes and a condenser connected between the control electrode of said second-mentioned tube and a tap in said path, the control electrode of said second mentioned tube being thus connected to derive its input potential from said tap in said impedance, said input potential thus being selected so as to be in opposed phase to said source of voltage variations and being dependent upon the varying potentials of the anodes of both of said two tubes.

3. In an electron discharge system the combination of an electron discharge tube having a control electrode and an anode, a source of voltage variations connected to said control electrode, a second electron discharge tube having a control electrode and an anode, a source of electrical supply connected in the output circuits of both of said tubes, a high impedance connected between the anodes of said two tubes, and a condenser connected between the control electrode of said second-mentioned tube and a tap in said high impedance located on the side of the center point of said impedance which is nearer the anode of said first-mentioned tube, the control electrode of said second-mentioned tube being thus connected to derive its input, in opposed phase to said source of voltage variations, from said tap in said high impedance.

4. In an electron discharge system the combination of an electron discharge tube having a control electrode and an anode, a source of voltage variations connected to said control electrode, a second electron discharge tube having a control electrode and an anode, a source of electrical supply connected in the output circuits of both of said tubes, a high impedance connected between the anodes of said two tubes, a condenser connected between the control electrode of said second-mentioned tube and a tap in said high impedance located on the side of the centre point of said impedance nearer the anode of said first-mentioned tube, and a further condenser connected between the control electrode of said first-mentioned tube and a tap in said high impedance at a point on the side of the centre point of said impedance opposite to that of first-mentioned tap, the control electrode of said secondmentioned tube being thus connected to derive its input, in opposed phase to said source of voltage variations, from said first-mentioned tap in. i

said high impedance.

5. In an electron discharge system, the combination of an electron discharge tube having a control electrode and an anode, a source of voltage variations connected to said control electrode, a second electron discharge tube having a control electrode and an anode, a source of electrical supply connected in the output circuits of both of said tubes, a high impedance connected between the anodes of said two tubes, a conductor connected between the control electrode of said second mentioned tube and a tap in said high impedance located on the side of the center point of said impedance which is nearer the anode of said first mentioned tube, and a condenser interposed between the control electrode of said second mentioned tube and said source of electrical supply, the control electrode of said second mentioned tube being thus connected to derive its input in opposed phase to said course of voltage variations, from said tap in said high impedance.

6. In an amplifying stage, two discharge tubes, each comprising a plate and a grid, and plate and grid circuits for each of said tubes, an input circuit connected to the grid and cathode of at least one of said tubes, a common impedance connected between the plates to receive the combined. varying output voltage of the two tubes, the grid of the other of said tubes being excited from said common impedance.

RUPERT EVAN HOWARD CARPENTER. 

